Method and system for recording of information on a holographic medium

ABSTRACT

There is disclosed a method for the holographic recording of data, wherein a hologram containing the date is recorded in a waveguide layer ( 3 ). The holograms are formed in a layer structure containing multiple waveguide layers ( 3 ). Coupling means ( 12 ) are provided for selectively coupling the reference beam ( 11 ) into the single waveguide layers ( 3 ) of the layer structure. The invention also relates to an arrangement for holographic recording of data, comprising a data storage medium with a waveguide holographic storage layer, and an optical system for writing and reading the holograms. The optical system comprises means for imaging an object beam ( 5 ) and a reference beam ( 11 ) on the storage medium. The arrangement comprises multiple waveguide holographic storage layers ( 3 ) in the storage medium, and means for selectively coupling the reference beam ( 11 ) into the single waveguide layers ( 3 ) of the layer structure.

TECHNICAL FIELD

[0001] The object of the invention is a method for the holographicrecording of data. In the method a hologram containing the date isrecorded in a waveguide layer as an interference between an object beamand a reference beam. The object beam is essentially perpendicular tothe plane of the hologram, while the reference beam is coupled in thewaveguide. There is also proposed an apparatus for performing themethod. The apparatus comprises a data storage medium with a waveguideholographic storage layer, and an optical system for writing and readingthe holograms. The optical system comprises means for producing anobject beam and a reference beam, and imaging the object beam and areference beam on the storage medium.

BACKGROUND ART

[0002] Storage systems realised with tapes stand out from other datastorage systems regarding their immense storage capacity. Such systemswere used to realise the storage of data in the order of Terabytes. Thislarge storage capacity is achieved partly by the storage density, andpartly by the length of the storage tapes. The relative spacerequirements of tapes are small, because they may be wound up into avery small volume. Their disadvantage is the relatively large randomaccess time.

[0003] The random access time may be decreased, or the capacity may beincreased with the same random access time, if the data storage is notonly done in the plane of the storage medium, but also in depth(so-called 3D or three-dimensional storage). Optical data storage offersseveral possibilities for 3D storage. One possible way is the solutionused in multi-layered CD-s, or the DVD. In this case the data storageplanes are spaced apart by some tens of μm-s. The applied optical systemhas a large numerical aperture, with a depth resolution of approx. 1 μm,and a precise focus servo system allows the selective readout of thelayers placed beneath each other.

[0004] Another solution is known from the area of holographic datastorage. In this case the data are stored as thick holograms (Braggholograms). Here the “depth addressing”, i.e. the separation of theholograms recorded into the same physical volume, may be achieved withthe Bragg conditions. This is known as angle-, wavelength-,displacement- etc. multiplexing. In the experimental holographic storagesystems in the laboratories primarily crystals are used as storagemedium (Fe doped LiNbO₃). This finds only limited applications, due toconsiderations of manufacturing technology, and may not be used at allfor tape storage systems. For this purpose only a polymer type materialis feasible.

[0005] Polymer based materials are normally produced in largequantities, relatively easily, and are easily fixed on a substrate.Known optical storage materials are the so-called side-chain polymers,and their use in holograms is also known. Another important property ofthese polymers is that no post-exposure treatment is necessary, e.g. nosubsequent development, thermal or electrical fixing process. This is avery important issue in all practical data storage technology.

[0006] It has been shown experimentally that so-called side-chainpolymers are excellently suitable for optical data storage purposes.Thin polarisation holograms may be recorded in side-chain polymers witha theoretical 100% efficiency. However, in order to record Braggholograms that are suitable for spatial (three-dimensional) storage, atleast 25-50 μm thick holographic storage material is necessary. Polymermaterials with such thickness undergo substantial deformation (as aresult of the change in temperature, mechanical impacts, humidity,etc.). The deformation of the holographic storage layer will cause thedeformation of the lattice in the hologram, and this will in turn leadto a decrease in diffraction efficiency. As the layer thicknessincreases, and the lattice deformation increases therewith, beyond acertain threshold the thick hologram will deteriorate, and finally itwill be unreadable. On the contrary, thin holograms are much lesssensitive to the deformation of the holographic lattice.

[0007] Therefore, it is an object of the present invention to provide astructure, which allows the recording of multiple holograms within thesame unit area of the data storage medium, and at least partlyeliminates the problems above. With the invention a data storagestructure is suggested, which allows in-depth data storage in case ofthin polymer storage materials. The suggested solution combines theadvantages of thin holograms (insensitivity to lattice deformation) withthe advantages of thick holograms (three-dimensional, in-depth storage,large data density).

SUMMARY OF THE INVENTION

[0008] According to the method of the invention, a hologram containingthe data is recorded in or in the vicinity of a waveguide layer as aninterference between an object beam-and a reference beam, where theobject beams is essentially perpendicular to the plane of the hologram,while the reference beam is coupled into the waveguide. According to theinvention, the holograms are formed in a layer structure containingmultiple waveguide layers, and coupling means are provided forselectively coupling the reference beam into the single waveguide layersof the layer structure.

[0009] In a preferred realisation of the method a grating is used as thecoupling means. Advantageously, gratings with different pitch and/orprofile are used in the different waveguide layers, and the referencebeam is projected at different incidence angles on the coupling means,i.e. the gratings.

[0010] Alternatively, gratings with different orientation may be used inthe different layers. In this case the reference beam is projected fromdifferent directions onto the coupling means, where the projections ofthe different directions projected on the plane of the data storagemedium are also different.

[0011] The invention also relates to an arrangement or system forholographic recording of data. The system comprises a data storagemedium with a waveguide holographic storage layer, and an optical systemfor writing and reading the holograms. The optical system comprisesmeans for producing an object beam and a reference beam, and imaging theobject beam and a reference beam on the storage medium. According to theinvention, the arrangement comprises multiple waveguide holographicstorage layers in the storage medium, and further comprises means forselectively coupling the reference beam into the single waveguide layersof the layer structure.

[0012] In a preferred embodiment, the optical system comprises means forprojecting the reference beam onto the coupling means from differentincidence angles. In this case it is particularly advantageous if thecoupling means comprises gratings with different pitch and/or profile inthe different layers.

[0013] In a further preferred embodiment, the optical system comprisesmeans for projecting the reference beam onto the coupling means fromdifferent directions, where the projections of these differentdirections in the plane of the data storage medium are also differentfrom each other, i.e. the components of the directions falling in theplane of the data storage medium point in different directions in thatplane. For this arrangement it is suggested that the coupling meanscomprises gratings with different orientation in the different layers.

[0014] In a further particularly preferred embodiment, it is foreseen touse a tape as the data storage medium. In order to facilitate the smoothreadout of the running tape, in a particularly preferred embodiment theoptical system is positioned in a rotating cylinder, where the cylinderis guiding the tape. In order to provide a continuous readout of thedata on the tape, in an advantageous embodiment there is providedmultiple optical systems within the cylinder.

[0015] The invention also concerns the waveguide structures used in theoptical system of the invention.

BRIEF DESCRIPTION OF DRAWINGS

[0016] The invention will be now explained in detail with reference tothe accompanying drawings, where

[0017]FIG. 1A illustrates the principle of the multi-layered holographicdata storage, and the multi-layered storage medium, the latter incross-section,

[0018]FIG. 1B is a modified version of the optical setup shown in FIG.1A

[0019]FIG. 2 is another modified version of the arrangement shown inFIG. 1A,

[0020]FIG. 2B is a modified version of the optical setup shown in FIG.2A

[0021]FIG. 3 is a schematic top view of the storage medium of FIG. 1,

[0022]FIG. 4 is a principal scheme of a read-write optical system usedwith the storage medium of FIGS. 1A-B and 2A-B,

[0023]FIG. 5 illustrates the working principle of another embodiment ofthe data storage arrangement of the invention, and

[0024]FIG. 6 illustrates the spatial arrangement of the elements of theoptical setup of FIG. 4, and

[0025]FIG. 7 is another modified version of the optical setup shown inFIG. 2A.

BEST MODE FOR CARRYING OUT THE INVENTION

[0026] The principal scheme of the multi-layered hologram structure isshown in FIG. 1A. The basic element of the system is a so-calledwaveguide hologram. Essentially, this means that the hologram isrecorded in a waveguide layer 3. The waveguide layer 3 is made of alight sensitive storage material with approx. 0.5-2 μm thickness, whichis sandwiched between two spacer layers 4. The thickness of the spacerlayers 4 is approx. 5-10 μm, and their refractive index is smaller thanthat of the waveguide layer 3. The system will work well if thedifference between the refractive indices—usually denoted by Δn—is inthe order of 0.01 and 0.001. There may be as much as 10-30 waveguidelayers and spacer layers stacked on each other, but only three waveguidelayer 3 is shown in FIG. 1, for simpler illustration. The waveguidelayers 3 are separated by two spacer layers 4 in FIG. 1. The externalsurface of the layer structure, i.e. the top waveguide layer 3 iscovered by the protective layer 7, while there is a mirror 8 under thebottom waveguide layer 3. The whole layer structure is supportedmechanically by the substrate 10.

[0027] A common object beam 5 is used for recording the holograms in thewaveguide layers 3. This object beam 5 is the Fourier transformed imageof spatially modulated light beam. The modulation is made by a spatiallight modulator (SLM) 6. The object beam 5 falls through a beam splitter9 perpendicularly onto the layer system consisting of the waveguidelayers 3 and the spacer layers 4. The layer system is placed in theFourier plane, or at least in its vicinity. The same object beam is usedfor all layers. With other words, the object beam 5 travels through allwaveguide layers 3, i.e. through all light sensitive layers.

[0028] The reference beam 11 is a guided plane wave travelling in one ofthe waveguide layers 3 of the layer system. One of the basic ideas ofthe invention is the provision of coupling means for selectivelycoupling the reference beam 11 into the individual waveguide layers 3.According to the invention, in one preferred embodiment this is made bycoupling the 11 reference beam into the selected waveguide layer 3 by anappropriately formed grating 12. The gratings 12 in the waveguide layers3 beneath each other all have different periodicity (pitch), i.e. thedistance between two lines of the grating is different. The selection oraddressing of the holograms 13 placed beneath and above each other ismade by selecting a different incidence angle a for the reference beam11 relative to the plane of the substrate. The incidence angle a willalso be termed as reference angle hereinafter. To a grating 12 with agiven periodicity there is a given reference angle, which will result inan efficient coupling into the waveguide layer, and will result in aneffective guided plane wave. The energy coupled into the other waveguidelayers 3 is several orders less.

[0029] The waveguide layers 3 may be single mode or multi-modewaveguides, they may all have the same thickness or they may also haveslightly different thickness. If there are single mode waveguide layerswith equal thickness, there is a grating with a different periodicity(pitch) and/or different profile for each waveguide layer. If there aremultimode waveguide layers with slightly different thickness, theperiodicity of the coupling gratings 12 may be the same, and in thiscase the selection or addressing of the given waveguide layers 3 is madeby selecting a proper incidence angle a of the reference beam 11, whichlatter is imaged on the layers by an objective lens 17. This systemallows simpler manufacture of the gratings, but the profiles of thegratings 12 can not be optimised for a given coupling direction. Thismay result in the decrease of the coupling efficiency.

[0030] The incidence angle a of the reference beam 11 may be varied byseveral methods. Firstly, it is contemplated to use an optical systemwith a mechanically positioned objective lens 17, to which the necessarylight power is guided by an optical fibre bundle. Alternatively, theoptical system used in the arrangement of the invention could have aseparate objective lens 17 for the different reference beams 11 having adifferent incidence angle a, and the necessary light power could becoupled to all objective lenses 17 from the common coherent lightsource. In this case the light beams through the not used objectivelenses 17 may be switched off by a controlled optical switch (notshown).

[0031] The simplest method for producing the coupling gratings 12 may beby pressing or rolling the appropriate grating profile into the spacerlayers 4 or into the waveguide layers 3. This process is very similar tothe manufacturing technology of security holograms. When produced inlarge quantities, the manufacturing of the gratings 12 may be done verycost-effectively.

[0032] During recording, a 13 hologram will be formed in the lightsensitive waveguide layer 3, as an interference pattern between theobject beam 5, falling perpendicularly to the plane S of the datastorage medium, and between the reference beam 11, which is guided formthe side into the waveguide layer 3. The light intensity will result ina change of the transmission or the refractive index of the lightsensitive storage material. In the first case we speak of a so-calledamplitude hologram, while in the second case a so-called phase hologramis recorded. If the change in the refractive index or transmissioninduced by the light beam is also dependent of the polarisation state ofthe light beam, then a so-called polarisation hologram may be recordedin the storage medium. If the waveguide layer 3 is made from a materialthat is sensitive to the polarisation of the light, then the object beam5 and the reference beam 11 are constituted by two mutuallyperpendicularly polarised light beams.

[0033] Various polymers are particularly suitable for recording thewaveguide holograms 13. The advantage of polymers is that they do notneed any kind of post-treatment, development, fixation, thermal orelectrical recording after the exposure of the holograms 13, as opposedto many other holographic storage materials. Polymers may be producedcost-effectively in large quantities. The matching of the differenttypes of polymers for the creation of the layer system is a relativelyeasy task (e.g. the substrate 10, the spacer layer 4, the waveguidelayer (storage layer) 3 and the protective layer 7 may all be made ofdifferent types of polymers). The so-called side chain or liquid crystalpolymer materials are particularly suitable for the recording ofpolarisation waveguide holograms.

[0034] Since the object beam 5 will travel during writing through allwaveguide layers 3, it will also modify or change those layers as well,into which the reference beam 11 actually is not coupled into. Inerasable and re-writable holographic storage materials the object beam 5will erase to a small extent the previously stored holograms in thelayers which are momentarily not addressed by the reference beam 11.This effect is similar to the effect experienced when holograms aremultiplexed.

[0035] For multiplexed holograms various exposure techniques have beendeveloped to prevent the erasure of the previously recorded holograms.These are mostly based on the over-exposure of the holograms recordedearlier, while the newer holograms are recorded with a continuouslydecreasing energy, for gradually diminishing the erasing effect. In theend, when all holograms have been recorded, all holograms will haveapproximately equal diffraction efficiencies. Depending of the type ofthe holographic material, the exposure strategy and the allowed spread(variation) of the diffraction efficiency, the number of multiplexedholograms may be between several tens and several thousands. In case ofthe waveguide holograms 13 recorded in the multi-layer structureaccording to the invention, essentially the same factors limit theuseful number of layers, namely the erasing effect when the multiplexedholograms are recorded. The maximum allowable number of the waveguidelayers 3—i.e. the number of multiplexed holograms recorded in the samearea of the layer structure—may be optimised by applying a specialexposure strategy tailored to the properties of the applied storagematerial. In case of the suggested side-chain polymers, approximatelyfive to ten layers may be placed on each other with the present level oftechnology.

[0036] During readout of the data recorded with the method of theinvention, the coupling grating 12 is illuminated with an appropriatelydirected reference beam 11. The guided reference beam 11 in thewaveguide layer 3 will diffract on the stored waveguide hologram 13, andwill create a light beam having the same properties as the object beam 5used during the writing stage, i.e. the diffracted reference beam 11will re-create the Fourier-transform of the light intensity distributioncreated by the SLM 6. The mirror 8 under the layer structure willreflect the re-created Fourier-transform into the same objective lens 14which was also used in the writing step. The read image is coupled outonto a detector matrix 15 by a beam splitter 9, which is typically thesame beam splitter that was also used in the recording process.

[0037] With an other possible embodiment shown in FIG. 1B, there is nomirror 8 under the layer system, and the substrate 10 is transparent.The diffracted readout beam 26—i.e. the reconstructed Fourier transformof the light beam which passed through the SLM—passing through thetransparent substrate 10 is transformed back by another Fourierobjective 27 and images the readout beam 26 on a detector matrix 15positioned on the other side of the substrate 10.

[0038] Both in the latter described transmission mode and in thereflection mode described in FIG. 1A, the layer system may be realisedslightly differently. This modified layer structure is illustrated inFIG. 2A and 2B. The arrangement shown in 2A has a substrate with amirror layer 8, while the arrangement in FIG. 2B is one with atransparent substrate, where the readout detector 15 is opposite to theside of the incident object beam 5. In this case the waveguide layer 23is made of a material that is not sensitive to light. The holograms arerecorded in a light sensitive layer 24 adjacent directly to thewaveguide layer 23. With other words, in this case the hologramcontaining-the data is recorded not in the waveguide layer directly, butonly in its vicinity. The light sensitive layer 24 may be on both sidesof the waveguide layer 23, or on just one side, as it is shown in FIG.2A and 2B. As previously, the reference beam 11 travels essentially inthe waveguide layer 23. However, in the vicinity of the waveguide layer23 an exponentially decreasing electromagnetic field will be formed,which is also termed as evanescent wave. This evanescent wave 25 extendsinto the thin light sensitive layer 24 situated directly beside thewaveguide layer 23 for a distance in the order of the appliedwavelength. With other words, the evanescent wave 25 will enter theactual storage material, the light sensitive layer 24. At the same time,the object beam 5 will travel perpendicularly to the layers, as in theprevious case. The interference pattern between the object beam 5 andthe evanescent wave 25 extending from the addressed waveguide layer 23into the light sensitive layer 24 will record a hologram in the lightsensitive layer 24. Depending on the chosen storage material, thehologram may be an amplitude-, phase-, or polarisation hologram.

[0039] When recording a hologram with a reference beam 11 utilizing theprinciple of the evanescent wave 25, the layer system may be made of twoor three different materials. With three different materials, thewaveguide layer 23 has the largest refractive index, and a thickness of1-2 μm. The light sensitive storage layer 24 is approx. 1-2 μm thick,while the spacer layer 4 is approx. 10 μm thick- The refractive index ofthe latter two layers is smaller than that of the waveguide layer 23.This layer structure has the advantage that the key properties of thelayers (waveguide, spacer, storage layer), and the material constantsmay be optimised separately.

[0040] A practical realisation of the method according to the inventionis illustrated with reference to FIGS. 3 and 4. FIG. 3 shows thestructure of the storage medium of a tape-based optical storage system,i.e. the structure of an optical tape 30, while FIG. 4 shows anarrangement suitable for the writing and reading of data to and from thetape 30 shown in FIG. 3.

[0041] The multi-layered waveguide holograms suggested by the inventionare particularly well suited for optical tape storage systems with alarge storage capacity. In a possible embodiment the holograms 13 withineach layer are positioned beside each other in multiple lines 31,extending along the length of the tape 30, i.e. there are several linesparallel to each other in the width of the tape 30. As an example, twolines 31 are shown in FIG. 3. When the holograms 13 are positioned inseveral lines 31, the number of the write/read heads 33 may be equal tothe number of lines 31. In this case the write/read heads 33 may beshifted relative to each other along the length of the tape 30.

[0042] Considering that the Fourier-plane is theoretically invariant tothe displacements in the X-Y direction (the displacements in the planeof the 13 hologram), with an optical system having appropriate imagingproperties it is allowable to shift the optical axis of the write/readhead 33 relative to the theoretical centre of the hologram 13 actuallybeing read, with a distance corresponding to approx. one tenth of thesize of the hologram (its size in the plane of the hologram, i.e. onetenth of its width or length). This is important because this means thatthe holographic data storage system proposed by the invention do notneed special manufacturing and positioning tolerances, and do not needany complicated servo system that would be otherwise required to producean exact tracking of the lines 31. During readout, the tape 30 need notbe stopped for each hologram 13, but the tape 30 may move continuouslyrelative to the write/read head 33.

[0043] With another possible embodiment, a single write/read head 33 mayread several lines 31. In this case the write/read head 33 must beprovided with positioning means, for movement perpendicularly to thelongitudinal direction of the tape 30.

[0044] The individual holograms 13 may be positioned so as to cover eachother completely when viewed from above. If the holograms covercompletely, the tape 30 must stop relative to the write/read head 33during both reading and writing. However, with a different approach, itis also possible for the tape 30 to move continuously at least duringreading. The readout is effected with a short laser pulse, which startswhen the hologram 13 to be read is exactly at the optical axis of thewrite/read head 33. The holograms 13 above and under each other areslightly shifted relative to each other in the longitudinal direction ofthe tape 30, as it is also perceived from FIG. 3. The amount of theshift is equal to the distance covered by the moving tape during thereadout time (i.e. the duration of the readout pulse). In this mannerwhen the next image is read, the next hologram 13 is positioned in thefield of view of the optical system.

[0045] Even in this arrangement the write/read head 33 and the tape 30must stand still relative to each other during writing. This arrangementprovides the advantage that the tape may move continuously duringreadout, and there are no sudden stops and large accelerations, and theoverall mechanical stress on the tape is smaller. Since the writing is aslower process than the reading, the mechanical stresses are smalleralready.

[0046] According to the invention, a further arrangement is proposed forsubstantially increasing the writing speed. This is achieved with anoptical system where the tape 30 need not be stopped during writing.This suggested embodiment is also advantageous because the continuousmovement will not impose hard wear on the tape 30. For this purpose, thewrite/read head 33 and the area of the tape being written must bestationary relative to each other during the writing process. Onepossible solution is to mount the write/read head 33 within a rotatingdrum or cylinder 34. This arrangement is shown schematically in FIG. 4.The tape 30 is guided by the periphery of the cylinder 34, withoutslipping. The wall of the cylinder 34 is transparent under the objectiveof the write/read head 33. The object beam leaving the objective and thereference beam leaving the separate reference optics enters the tape 30,which latter is standing still relative to the objective lens (in fact,they rotate together). As a result, the object beam and the referencebeam will produce a clear, well-defined and distortion-free interferencepattern, and the stored holograms will be well readable.

[0047] As it is seen in FIG. 3., for one half of the holograms 13 aboveeach other on the tape 30, the coupling gratings 12 are assigned fromthe right side, while to the other half from the left side. In thismanner the number of holograms 13 that may be addressed independentlyfrom each other with a good coupling efficiency may be twice the numberof the different coupling gratings 12. This means that assuming alimited entry angle interval for the reference beams, the selectivity ofthe reference beams is better, with other words, the ratio between theintensity coupled into the selected waveguide and the intensity coupledinto the neighbouring waveguides (i.e. the “parasitic” intensities dueto scattering effects) is better. With other words, the cross-talkbetween the holograms above each other will be less.

[0048] A tape-based optical storage system applying the inventiveprinciple may provide substantial data storage capacity. Feasible andexperimentally proven, relatively simple optical systems and existingstorage materials (e.g. the mentioned side-chain polymers) allow therealisation of approx. 1.5 bit/μm² data density. With off-the-shelf,available SLMs, (e.g. with 320×240 or 512×512 pixels), the size of onehologram could be a few tenth of a millimetre. Practically, four rows ofcoupling gratings may be arranged on an optical tape with 2 mm width.Calculated with the full width of the tape, the total data density couldbe approx. 1 bit/μm². With other words, in one millimetre length of thetape the capacity is approximately. 2000×1000=200 Kbytes. Assuming tenslayers above each other results in the theoretical storage capacity of 2Mbytes/mm tape. A 30 m long tape would have the capacity of approx. 60Gbyte. It must be noted that this theoretical value is the so-called rawcapacity. The necessary error coding, control bits, etc. will reduce thepractically useful capacity to 50-60% of the raw capacity, assuming theusual data coding algorithms and data organising methods (file anddirectory structure).

[0049] Finally, an optical storage system using a card or a disk ispresented with reference to FIGS. 5. and 6.

[0050] Beside the tape-based storage systems, the multi-layeredwaveguide holograms are applicable for other known storage systems aswell, e.g. for card or disk optical storage systems. The main parametersof the waveguide structure are essentially the same as shown in FIGS. 1or 2. The differences between the data storage mediums is found in therelative arrangement of the 13 holograms and the coupling gratings 12.The arrangement shown in FIG. 3 is optimal for tape storageapplications. With disk data storage devices the 13 holograms arearranged in a circles or along a spiral, in a usual manner. The properpositioning (tracking) of the readout optics is made by a knownopto-electronical and opto-mechanical servo system. This system is notdescribed here in detail, but it is known in the art.

[0051] For data storage systems having a card-type data storage medium,or an equivalent, essentially stationary storage medium, anothergrating-hologram arrangement is suggested. With such a data storagecard, the storage medium moves much slower than an optical tape. As aresult, the readout speed (data transfer rate) is lower. This isalleviated by the fact that normally it is not necessary to read a largeamount of data from the card storage mediums, due to the type of thedata stored (personal data, bank accounts, personal medical data, etc.).The data transfer rate may increase, if the holograms in the differentlayers are positioned exactly above each other, and not shifted as withthe tape system. After a positioning operation of the card actuatormechanism, a selected hologram in any of the layers may be reached witha fast change of the incidence angle or the direction of the referencebeam. The readout speed may be limited by the performance of the readoutelectronics as well. Therefore, it is preferred to use a readout opticsand electronics based on a fast CCD or CMOS photodetectors.

[0052] With a possible embodiment of the card-type storage medium thecoupling gratings 42 surround the holograms 43 stacked above each other(see FIGS. 5 and 6.). In this case the coupling gratings 42 associatedto the different layers are positioned at an angle to each other. Thismeans that all the gratings 42 may have the same profile and the sameperiodicity. This embodiment is inherently simple, and makes theproduction of the gratings much cheaper. The addressing of the selectedlayers is performed by rotating the reference beam 41 around an axisperpendicular to planes of the holograms 43, and going through thecommon centre of the holograms 43, while the reference beam 42 fallsonto the hologram 43 under a given incidence angle β. This perpendicularaxis of rotation practically coincides with the optical axis of theobject beam 45 (see especially FIG. 6. With other words, the referencebeam 41 is rotated around the optical axis of the Fourier-objective 14with a given tilting angle β. In this manner the reference beam 41 onlycoincides with one of the gratings 42 at one time, and therefore will becoupled into one layer only with good efficiency. At the same time, theprojection of the directions of the different reference beams 41associated to the different coupling gratings 42 projected onto theplane of the substrate 10 will be different. Note that in FIG. 6, theplane of the substrate 10 essentially coincides with the plane of thecard 46 itself. With other words, the differently directed gratings 42will act as the selective coupling means, co-operating with thereference beams 41 having the proper direction.

[0053] It is also possible to record the stacked holograms using anevanescent reference beam 11 with two types of repeating layers only.Such a waveguide structure is shown in FIG. 7. The optical system forrecording and readout is similar to that shown in FIGS. 1A and 2A. Thereare also provided selective coupling means, the gratings 12 for eachwaveguide layer 23. However, in this case there is no separate lightsensitive layer 24 and a spacer layer 4 in the waveguide structure, andthese two layers is substituted by a single light sensitive spacer layer29 with an approximate thickness of 10 μm. With other words, this systemmay be regarded as a waveguide structure where the spacer layer itselfis suitable for recording of holograms. Here the reference beam 11 willtravel in the waveguide layer 23, which is only 1-2 μm thick, and ismade of a material that is not sensitive to light. Because of theexponentially decaying evanescent wave 25 the hologram in the thicklight spacer sensitive layer 29 will be only formed in the directvicinity of the waveguide layer 23. If the storage material issufficiently thick, no coupling between the layers will arise.

[0054] The waveguide structure in FIG. 7 also comprises a mirror 8between the layer stack used for the recording and the substrate 10.However, it is also foreseen to use the waveguide structure of FIG. 7without a mirror layer 8, and with a transparent substrate 10. In thiscase, the optical system for recording and readout may be modifiedsimilarly to those shown in FIGS. 1B and 2B.

1. Method for the holographic recording of data, wherein a hologramcontaining the data is recorded in or in the vicinity of a waveguidelayer as an interference between an object beam essentiallyperpendicular to the plane of the hologram and a reference beam coupledin a waveguide, characterised by that the holograms are formed in alayer structure containing multiple waveguide layers, and providingcoupling means for selectively coupling the reference beam into thesingle waveguide layers of the layer structure.
 2. Method according toclaim 1, characterised by using a grating as the coupling means. 3.Method according to claim 2, characterised by using gratings withdifferent pitch and/or profile in the different waveguide layers, andprojecting the reference beam is in different incidence angles on thecoupling means.
 4. Method according to any one of claims 1-3,characterised by using gratings with different orientation in thedifferent layers.
 5. Method according to any one of claims 1-3,characterised by projecting the reference beam from different directionsonto the coupling means, where the projections of the differentdirections projected on the plane of the data storage medium are alsodifferent.
 6. Arrangement for holographic recording of data, comprisinga data storage medium with a waveguide holographic storage layer, and anoptical system for writing and reading the holograms, the optical systemcomprising means for producing an object beam and a reference beam, andimaging the object beam and a reference beam on the storage medium,characterised by comprising multiple waveguide holographic storagelayers in the storage medium, and further comprising means forselectively coupling the reference beam into the single waveguide layersof the layer structure.
 7. Arrangement according to claim 6,characterised in that the optical system comprises means for projectingthe reference beam onto the coupling means from different incidenceangles.
 8. Arrangement according to claim 7, characterised in that thecoupling means comprises gratings with different pitch and/or profile inthe different layers.
 9. Arrangement according to any one of claims 6-8,characterised in that the optical system comprises means for projectingthe reference beam onto the coupling means from different directionshaving different projections on the plane of the data storage medium.10. Arrangement according to any one of claims 6-9, characterised inthat the coupling means comprises gratings with different orientation inthe different layers.
 11. Arrangement according to any one of claims6-10, characterised by using a tape as the data storage medium. 12.Arrangement according to claim 11, characterised in that the opticalsystem is positioned in a rotating cylinder, where the cylinder isguiding the tape.
 13. Arrangement according to claim 12, characterisedin that there is provided multiple optical systems within the cylinder.14. A waveguide layer structure for holographic storage, comprising asubstrate and at least one holographic storage layer sensitive to light,and further comprising multiple waveguide layers on the substrate, withspacer layers between the waveguide layers, characterised by comprisingmeans for selectively coupling the reference beam into the singlewaveguide layers of the layer structure.
 15. The waveguide structureaccording to claim 14, characterised in that the coupling meanscomprises gratings with different orientation in the different waveguidelayers.
 16. The waveguide structure according to claim 14 or 15,characterised in that the coupling means comprises gratings withdifferent pitch and/or profile in the different waveguide layers, 17.The waveguide structure according to any one of claims 14-16,characterised in further comprising a mirror layer between the substrateand the waveguide layer adjacent to the substrate.
 18. The waveguidestructure according to any one of claims 14-16, characterised in thatthe substrate is transparent.
 19. The waveguide structure according toany one of claims 14-18, characterised in that the waveguide layer is alight sensitive layer suitable for the recording of holograms.
 20. Thewaveguide structure according to any one of claims 14-18, characterisedin further comprising at least one separate holographic storage layeradjacent to each waveguide layer.
 21. The waveguide structure accordingto any one of claims 14-18, characterised in comprising a spacer layerbeing suitable for recording of holograms.
 22. The waveguide structureaccording to any one of claims 14-21, characterised in furthercomprising a protective layer on an external surface of the layerstructure.